With increasing landfill costs and decreasing landfill availability, the cost of waste disposal has increased dramatically during recent years and is expected to increase further. This is a particularly serious problem for those, such as electric utility companies, who have large quantities of waste wood for disposal. Much of waste wood, however, has potential fuel value. If efficiently combusted, waste wood can provide dual benefits of alleviating landfill problems and producing needed energy. Waste wood is a relatively low cost fuel and, when efficiently combusted, emits relatively low levels of sulfur and ash.
Waste wood has long been burned in furnaces to produce energy. More efficient methods of converting waste wood to energy including comminuting waste wood to a reduced particle size having a maximum dimension from about one to four inches (2.5 to 10 cm) and burning the particulate wood fuel on grates in a boiler, in stoker furnaces, and in fluidized bed reactors. Such methods are effective to produce energy from waste wood but tend to be difficult to control because of variations in particle size, type, and quality of the wood being burned.
There exist a number of older, particulate fossil fuel suspension furnaces for boilers that produce electrical energy via the combustion of particulate fossil fuels such as pulverized coal and atomized oil. Natural gas may also be used in such furnaces in conjunction with the particulate fossil fuel. These suspension furnaces, which include tangentially-fired suspension furnaces and wall-fired suspension furnaces, are still used to some extent by utilities and manufacturing companies such as paper mills, but are relatively inefficient when compared to modern reactors that burn fossil fuels and other types of reactors such as nuclear reactors. Today, particulate fossil fuel suspension furnaces are typically used only in times of peak energy demand to supplement the primary electrical power system. Attempts have been made to more economically produce energy with suspension furnaces by burning wood in such furnaces along with fossil fuels, but those attempts have not been successful in burning wood at a high enough rate to economically produce energy. Because waste wood is inexpensive and available in large quantities, particulate fossil fuel furnaces could produce energy more economically if waste wood could be burned therein at a high enough rate. Thus, burning wood in particulate fossil fuel suspension boilers could increase the usefulness of such boilers and add to their value.
Prior attempts to burn wood in particulate fossil fuel suspension furnaces include co-firing waste wood particles with pulverized coal by mixing the waste wood particles with the coal at the coal pile and introducing the combined material into the furnace through the coal pulverizers which normally pulverize and then deliver pulverized coal to the furnace. This method can be used with combined fuels that contain less than about 10% by weight of wood particles, but such an amount of wood particles is ineffective to more economically produce energy with the furnace than by burning solely fossil fuels. When greater than about 10% by weight of particulate wood was mixed with the coal and fed to the pulverizer, the wood particles inhibited the pulverization of the coal such that the mixed fuel could not be efficiently combusted in the furnace.
Therefore, there is a need for a particulate waste wood fuel and a method for burning such fuel in a particulate fossil fuel suspension furnace to economically produce energy and increase the usefulness of such suspension furnaces and associated boilers.